It sounds like you can disable Metrics in the settings.
BTW, I'm less concerned and more just surprised to see Google Analytics used like this. I guess it really is the blurring of the lines between desktop and browser-based apps.
Maybe this GA usage is not unique, but it's new to me.
You are, though it's mentioned on the very first screen that pops up when you open the app: https://github.com/atom/welcome/blob/master/lib/welcome.md